An X-ray generator for generating ion-gas by X-irradiating air is disclosed in Japanese Patent Publication 2005-116534. The X-ray tube used for the X-ray generator is formed of a cylindrical package or a bulb as a main body. In the package, electrons emitted from a filament is focused and subjected to collide with an X-ray target so that the X-ray is generated. Then, the X-ray passes through an X-ray transmissive output window, and exits outside of the package.
FIG. 8 is a cross-sectional view of round shaped X-ray tube similar to the X-ray tube explained herein above. The round shaped X-ray tube includes a main body formed of a cylindrical package 100 made of glass. On one side of the cylindrical package 100, a circular opening is formed which is closed by an X-ray transmissive window 101 consisting of a beryllium film, and the inside of the cylindrical package 100 is maintained in a high-vacuum state. On the inside surface of the X-ray transmissive window 101 in the package 100, an X-ray target 102 is arranged. In the other side of the cylindrical package 100, a cathode 103 as an electron source and a control electrode 104 are arranged. Electrons emitted from the cathode 103 is accelerated and forced by the control electrode 104, and collides with the X-ray target 102, and the X-ray is emitted from the X-ray transmissive window 101 to the outside of the package 100. Furthermore, as shown in FIG. 8, the X-ray emitted from the X-ray transmissive window 101 to the outside of the package 100 is graphically indicated by the mark X, and the center of X-ray emission in the X-ray transmissive window 101 is shown by the mark P.
In the conventional X-ray tube shown in FIG. 8, electrons emitted from the cathode 103 bring into focus in the shape of a beam, namely, a dot-shaped X-ray irradiation extending radially from the focus point P on which the electrons collide with the target 102. After the X-ray exits from the X-ray transmissive window 101, the X-ray is expanded like a cone shape as shown by the mark X in FIG. 8. Accordingly, an effective irradiation area is smaller than the size of an irradiation object. Thus, in order to x-irradiate over a wide range by using the round shaped X-ray tube having small the irradiation area, it is necessary to use a large number of X-ray tubes, and arrange them in an array. As a result, the round shaped X-ray tube is not totally satisfactory from a standpoint of facility and maintenance costs. Furthermore, in order to x-irradiate over a wide range, it may be possible to x-irradiate away from an object. However, it is required to increase irradiation intensity so as to x-irradiate the irradiation object in this instance, and an unnecessary area is x-irradiated. As a result, X-ray leakage problem is caused.
Accordingly, in order to solve the above problem in the conventional round-shaped X-ray tube, the inventers of the present invention invented a flat-shaped X-ray tube as shown in FIGS. 6 and 7. The X-ray tube comprises a package 55 which includes a substrate 53 made of radiopaque metal attached to an open-side periphery of a case 51 made of glass plates and formed in a box shape. On the substrate 53, a slit-shaped opening 52, as for example, about 2 mm in size is formed. In the package 55, an X-ray transmissive window 54 made of a titanium foil is attached to the opening 52 from the outside of the substrate 53 so that the opening 52 is closed. The inside of the package 55 is maintained in a high-vacuum state. In the package 55, a target 56 made of tungsten is arranged on the X-ray transmissive window 54 located on the opening 52 of the substrate 53. Furthermore, in the inside of the package 55, a back plate 57 is arranged on an inner surface opposite to the X-ray transmissive window 54. On the lower side of the back plate 57, a filamentary cathode 58, a first control electrode 59 deriving electrons from the cathode 58, and a second control electrode 60 accelerating the electrons derived by the first control electrode 59 are sequentially arranged.
According to the above X-ray tube, the electrons derived from the cathode 58 by the first control electrode 60 is accelerated by the second control electrode 60, and the X-ray generated by colliding with the X-ray target 56 penetrates through the X-ray transmissive window 54, and is emitted to the outside of the package 55. In this manner, titanium having good radiolucency and high intensity as material of the X-ray transmissive window 54 is used for the X-ray tube. The X-ray tube does not use beryllium which becomes hazardous by oxidation. Furthermore, since the X-ray is emitted from the X-ray transmissive window 54 regulated by the opening 52 of the substrate 53, the X-ray can be projected with a silt width of the X-ray transmissive window 54 substantively in a liner pattern over, the area in which the X-ray is radiated, if the size of the opening formed in the elongated slit-shaped is set in a desired size. Thus, it is possible to easily set the irradiation area of effective dimension in a relatively high freedom degree corresponding to the size of the object contrary to the X-ray tube in which the irradiation area is small and formed in round shape. In addition, if the size and shape of the opening 52 are formed with a desired size in the shape of rectangular groove, the area in which the X-ray is emitted in the X-ray transmissive window can be easily recognized from the outer shape compared with the circular X-ray transmissive window. Thus, it is relatively easy to set a pathway precisely guiding the X-ray in a predetermined position.
However, according to the flat shaped X-ray tube shown in FIGS. 6 and 7 proposed by the inventors of the present invention, the substrate 53 made of metal and the X-ray transmissive window 54 made of a titanium foil arranged on the opening 52 of the substrate 53 are deformed by external pressure resulting from vacuum atmosphere of the package 55. In some cases, the substrate 53 and the X-ray transmissive window 54 are broken to destroy the vacuum state in the package. Moreover, since the mechanical strength of the X-ray transmissive window 54 made of a titanium foil is low, the X-ray transmissive window 54 is damaged with a sight power, it is hard to arrange the X-ray transmissive window on the substrate 53, and the substrate 53 requires careful handing after attaching the X-ray transmissive window 54 to the substrate 53.
In view of problems of the flat shaped X-ray tube having an electron source and an X-ray target within a package explained hereinabove, an object of the present invention is to provide an X-ray tube which improves strength of the package near the X-ray transmissive window radiating X-ray outside of the package, and can be easy to arrange the X-ray transmissive window on the substrate and to handle the substrate after forming the X-ray transmissive window.